Our objective is to elucidate the hereditary control of organelle structure and function in a eukaryotic cell by studying Mendelian (nuclear) and non-Mendelian (organelle) mutations affecting chloroplast and mitochondrial functions. Using the green alga Chlamydomonas reinhardtii as a model system, we plan to isolate and characterize mutations affecting the structure, assembly and function of chloroplast ribosomes. We are also looking for other mutations affecting the chloroplast, such as those controlling the chlorophyll biosynthetic pathway and coding for the CO2 fixing enzyme ribulose diphosphate carboxylase. Mutants defective in chloroplast functions in C. reinhardtii are conditionally viable, since chloroplast protein synthesis and photosynthesis are dispensable under appropriate growth conditions. Mitochondrial mutations to antibiotic resistance, and mutations affecting mitochondrial functions such as the respiratory electron transport pathway, which may survive under photo-synthetic conditions, will also be sought. In no other single species having physically defined genomes in its chloroplast, mitochondria and nucleus can one determine the dependence of both organelles on their own genomes, on that of the nucleus and each other's genomes. As a necessary part of these studies we will continue to examine the genetic basis for the uniparental transmission of chloroplast genes in crosses and to map the genomes of both the chloroplast and mitochondria as the isolation of new mutations permits.